1. Field of the Invention
It has been conventionally known to decompose organic wastes with supercritical water. It is also known that decomposition of organic materials with supercritical water results in a clean decomposition product, thereby decomposing even PCB, dioxine, etc.
The present invention relates to the technology of organic wastes decomposition with supercritical water, and more particularly to a method for treating organic wastes in which carbon contained therein is reacted with supercritical water under a specific condition to convert all of such organic wastes to a gas product containing hydrogen as a main component and a carbonate. The thus-formed decomposition product is quite dean as a matter of course.
2. Description of the Related Art
FIG. 1 shows a conventional oxidation decomposing method of organic materials by the use of supercritical water.
After organic materials and water are mixed, the mixture is pumped to a supercritical water reactor 1 where water in the supercritical state is formed by means of a pump 2, kept in the supercritical water reactor 1 at a temperature of about 400.degree. C. for a predetermined period of reaction time and then cooled by means of cooler 3 under a reduced pressure to form a gaseous oxidation decomposed product, water and solid residue. Air or oxygen as an oxidizing agent is supplied to the supercritical water reactor 1 to maintain the supercritical state while externally heating the reactor if the heat generated therein is not enough.
Under the above mentioned reaction conditions, the gaseous oxidation decomposed product predominantly comprises carbon dioxide and nitrogen gas originating from nitrogen components contained in the organic materials. It is well known that dioxins causing a problem in general incineration is not produced at this time.
According to a conventional process, thermal decomposition is conducted in supercritical water at a temperature of 400 to 600.degree. C. only with external heating without supplying any oxidizing agent, which results in a gaseous thermal decomposition product containing carbon monoxide, carbon dioxide, lower hydrocarbons, hydrogen and nitrogen as a main component and, in many cases, a liquid product. As the temperature of thermal decomposition increases, the yield of the gaseous product increases but that of hydrogen decreases.
According to JP-A No. 2-274,269, flon or flon-containing material is decomposed by alkali-containing supercritical water to control environmental pollution. In this process, carbon contained in organic wastes does not react quickly when oxygen or air is used as an oxidizing agent at a reaction temperature of 400.degree. C.
On the other hand, JP-A No. 9-314,155 discloses a process in which organic materials contained in sewage are combined with an oxidizing agent and introduced to a supercritical zone to decompose an organic component thereof by critical oxidation. When salts formed during the decomposition are dissolved in semi-critical water in a semi-critical zone and discharged as an effluent from a reaction system, the effluent is exhausted as it is if the content of the organic materials is within a predetermined standard level, while it is recycled to the reaction system if such an organic content is above the level.
According to this reference, however, reaction with supercritical water "without substantially adding an oxidizing agent" is not disclosed.
It has been conventionally conducted to decompose organic materials by oxidation in supercritical water while adding an oxidizing agent, or without adding any oxidizing agent for a purpose to obtain elements or molecules as constituents of the organic materials. However, it has not been thought or proposed to react carbon contained in organic materials with supercritical water followed by reductive decomposition of the supercritical water and simultaneous decomposition of the organic materials.
From the above mentioned viewpoint, the inventors have exclusively investigated various conditions to thermochemically decompose water and at the same time to decompose organic materials, and finally found that such decomposition can be achieved by a combination of supercritical water at a specific condition and a carbon dioxide absorbing substance without adding an oxidizing agent.